This invention has utility in industrial and/or scientific applications involving heat transfer or heat exchange. One typical use occurs in the operation of power plants. In such operations, large quantities of operating fluids, often at high temperatures, need be cooled, i.e., need have heat removed. A presently used mode of heat removal involves the circulating of transfer or working fluid through a large number of tubes. The operating fluids are caused to flow by the working-fluid tubes, permitting the exchange of heat between such fluids. Any event that reduces such heat exchange is deleterious to the process. While this example involves exchanging heat from an operating fluid to a cooling working fluid, it is unimportant to this invention the direction of heat exchange, i.e., from operating to working fluid, or vice versa. Of particular concern are [1] the forming of a contaminant layer on the inside tube wall, and [2] the forming of a thin annular, fluid film, sometimes described as a laminar film, of stagnant working fluid, just radially interior of the tube wall. Each of these disruptants apparently tends to reduce the exchange of heat between the adjacent fluids, i.e., acts as a heat insulator. Numerous approaches have been used to overcome these problems, such as the chemical and/or mechanical cleaning of the tube. Although a patent search has not been performed, applicant is aware of the "cleaning ball" system of Amertap Corporation, as described in its "An engineering staff report", and the brush cleaning system described in the September, 1975 issue of Heating/Piping/Air Conditioning published by Water Services of America, Inc. The former system utilizes sponge rubber balls, flowing in a closed circulation system, to clean the tube interior. The latter system includes cleaning brushes movable in a longitudinally extending tube. The direction of movement of the cleaning fluid may be reversed, so as to cause the brushes to periodically traverse the length of the tube. Applicant's system seeks to improve on the tube-cleaning systems described above.